Several types of labelling apparatus are available which apply glue to labels and subsequently apply them to conveyed containers. An example of such labelling apparatus is disclosed in Caulford et al, U.S. Pat. No. 3,450,586 and Carter, U.S. Pat. No. 3,864,187. The principal problem to be avoided in applying glue to labels is to prevent application of glue to the bare face of the pad for holding a label, should a label not be present. This can result in considerable down time on the machine, since the line has to be stopped, the glue removed from the label carrier device and then the system restarted. To avoid this problem, many attempts have been made to withdraw or avoid application of glue to the label carrier in the absence of a label. U.S. Pat. No. 3,450,586 discloses a device which senses whether or not a label is present for transfer onto a vacuum pad which holds and carries the label for glue application. If a label is not present on the transfer device, the vacuum pad is withdrawn by use of a cam latching mechanism to avoid contact with the glue applying device.
Similarly with U.S. Pat. No. 3,864,187, in the absence of a label on a vacuum pad which supports the label for application of glue, the pad is withdrawn. The vacuum pad is normally biased to its extended position so as to always contact the glue applicator. In the absence of a label on the pad, there is loss of vacuum which causes a cam follower to drop downwardly from the pad and engage a cam to withdraw the pad to avoid contact with the glue applicator. This approach necessitates the use of camming devices which often require adjustment because of wear.
Use may be made of photoelectric sensors to determine the presence of a label on a vacuum pad for holding a label to be glued. An example of this system can be found on an automatic labeller sold by Stackpole Machinery Company of Scarborough, Canada under the trademark SM 1700. With that device, a photoelectric cell determines if the label has or has not been transferred to a vacuum pad. Upon sensing the absence of a label, an air cylinder is actuated to move a cam into position which retracts the pad by way of a cam follower attached to the pad, so as to avoid contact with the glue applying station. The pad is, however, in its normal position urged outwardly to its extended position which contacts the label with the glue applying device.
Other approaches, which have been used in either withdrawing the carrier for a label to which glue is to be applied, or retracting the glue applicator, are disclosed in U.S. Pat. Nos. 3,112,236; 3,322,597; 3,982,472 and 4,242,167. In Canadian patent 502,685, a system is disclosed for retracting the glue roller for applying glue to a label on detecting the absence of a label on the vacuum pad. The absence of the label is sensed by a loss of vacuum on the vacuum pad and, in turn, actuates the device for retracting the glue roller out of the path of the vacuum pad. Retraction of the glue roller is rather cumbersome because of its size and the amount of glue the applicator usually carries.
The above systems function satisfactorily in a reasonably clean environment; however, with mechanisms which involve mechanical arrangements to retract the pad or retract the glue applicator to avoid application of glue to a bare pad, the devices may wear and require adjustment. A further consideration is that the devices normally work in a dusty environment caused primarily by minute paper particles working their way into the various pneumatic systems. For example, with the system of U.S. Pat. No. 3,864,187, minute paper particles may collect in the cylinder arrangement to obstruct the free fall of the cam in the absence of vacuum in the chamber. As a result, the pad remains in its outermost position and glue is applied to the bare pad. Systems which rely upon photoelectric sensing are not always reliable, because of the short time within which the electric eye must sense the presence or absence of a label. The sensors are subject to electronic failure and variation in sensitivity which can also result in application of glue to a bare pad. Another consideration is that with many of the systems the presence of a label is sensed before transfer to the pad. There is not an absolute sensing of whether or not a label is present on the pad. This may happen in the improper transfer of label to pad where the label covers only a portion of the pad. The pad can remain in its extended position and have glue applied to the exposed portion of the pad.
The method and apparatus, according to this invention, overcomes a number of the above problems by normally retracting the pad as it passes by the glue applicator and in response to vacuum being maintained on the pad due to the presence of a label, the pad is retained in its extended position so as to apply glue to the label for subsequent application to a conveyed container.